Because filters generally require some type of tuned circuit to provide frequency selectivity, it is very difficult and costly to incorporate filters into integrated circuits, or circuits on semiconductor chips. For example, a standard approach to realizing multi-octave tunable filters is to place tiny ferrite resonators in a magnetic field generated by a solenoid. The size and weight of these filters are primarily determined by the solenoid. The ferrite resonators are spheres which are mounted on rods for ease of handling and orientation. As such, the filters are bulky and very expensive. These structures are definitely not something to be considered for portable applications, such as paging, cellular telephones, etc.
Single crystal yttrium iron garnet (YIG) or gallium-substituted YIG (GaYIG) are magnetic insulators which resonate at a microwave frequency when magnetized by a suitable direct magnetic field. A unique feature of this resonance is that for a spherical YIG configuration, the resonant frequency is only related to the direct magnetic field and not to its dimensions. The basic ferrimagnetic resonance phenomenon can be explained in terms of spinning electrons which create a net magnetic moment in each molecule of a YIG crystal. This electron precession may be used to couple two orthogonal circuits at a microwave signal frequency equal to that of the precession. Using this phenomenon, current controlled tunable microwave filters have been constructed. Multi-octave tuning is in fact readily achieved with such resonators in the 500 MHz to 40 GHz range.
The unloaded Q-factor of these resonators is related to the magnetic and dielectric dissipation (loss tangents) within the YIG material. These losses are fortunately very low. Unloaded Q-factors of the order of 10,000 are realizable using highly polished YIG spheres. Such a value of unloaded Q-factor is indeed nearly as good as that obtainable using conventional waveguide cavities.
The low frequency limit of a YIG resonator is established by the fact that as the frequency is reduced the direct magnetic field required for ferrimagnetic resonance becomes insufficient to align all the magnetic dipoles within the crystal. In this instance, each dipole exhibits a separate resonance absorption, even in the absence of a direct magnetic field. The frequency at which this type of loss first occurs is determined by the magnetization and shape demagnetization of the YIG resonator. The magnetization is reduced through substitution of iron in the YIG crystal with a non-magnetic element such as gallium (GaYIG). Although the linewidth of the GaYIG described is not as good as that of pure YIG, satisfactory operation (using a sphere) is possible at frequencies as low as 360 MHz.
YIG resonators exhibit non-linear microwave losses (limiting) at large signal levels due to the transfer of energy from the uniform mode of magnetization to the so-called spin wave modes. In the usual YIG filter arrangement, first and second order instabilities under perpendicular pumping must be considered separately. In the first order instability (coincidence limiting), the frequency of the pump is twice that of the spinwave mode, whereas, in the second order instability (premature decline limiting), the two frequencies are equal. Coincidence limiting is frequency selective and occurs over a well-defined frequency interval defined by ##EQU1## where: .gamma.=the gyromagnetic ratio equal to 2,21*E5 (rad/s) (A/m),
M.sub.0 =the magnetization, PA1 .omega.=the radian frequency, PA1 N.sub.t =transverse demagnetization factor, and PA1 .mu..sub.0 -the free space permeability.
For pure YIG spheres (N.sub.t =1/3) the frequency interval for coincidence limiting lies between 1,660 and 3,320 MHz. The threshold power for coincidence limiting is particularly low and occurs at power levels between -15 dBm and -20 dBm. The critical magnetic field is mainly determined by the uniform and spinwave linewidths and the magnetization of the garnet material. To operate a spherical resonator outside of coincidence limiting a lower ferrite magnetization is necessary. Lower ferrite magnetization means lower Q-factor.
The maximum volume of a YIG sphere is fixed by the excitation of higher order magnetostatic modes within the YIG resonator. The minimum volume is set by the degradation of the unloaded Q-factor due to scattering of the uniform mode into so-called spinwaves via surface irregularities. YIG spheres normally have radii between 0.5 mm and 1.0 mm.
A solid is said to be in the crystalline state if its constituent atoms or groups of atoms are arranged in an angular, periodic array. In a magnetic single crystal the magnetization tends to be directed along certain definite crystallographic axes which, accordingly, are called directions of easy magnetization; the directions along which it is most difficult to magnetize the crystal are called hard directions. Experimentally, it is found that it requires the expenditure of a certain amount of energy to magnetize a single crystal to saturation in a hard direction. The difference between this energy and that required to saturate the crystal along a direction of easy magnetization is known as the anisotropy energy.
Magnetic anisotropy energy modifies Kittel's resonance condition and so this quantity must be recalculated. Since the crystalline energy is dependent upon the orientation of the crystal, the resonant frequency will be dependent upon its orientation in the external direct magnetic field. It is therefore, essential, in a multi-resonator filter to make provisions to align all of the resonators along the same crystallographic axis. It appears from experiments that in most cases the crystal anisotropy is very dependent upon temperature. Consequently, the reasons for mounting the spheres on individual rods, for tweaking purposes, so that the crystal can be oriented in the magnetic field so that temperature effects can be minimized.
Thus, it would be highly desirable to provide ferrite resonators that are small enough to be used in portable devices and especially in portable communication devices.
Accordingly, it is a purpose of the present invention to provide new and improved ferrite resonators which are small enough to be used in portable communications devices.
Further, it is a purpose of the present invention to provide new and improved ferrite resonators which are relatively inexpensive to manufacture.
It is a still further purpose of the present invention to provide new and improved ferrite resonators which are relatively easy to manufacture and to incorporate into high quantity production.
It is another purpose of the present invention to provide new and improved ferrite resonators which allow filters and the like to be integrated into associated circuits on a single chip.